In CW navigation systems such as OMEGA, DECCA, and LORAC receivers determine relative RF-phase of synchronized radio signals by amplifying the signals separately with either time or frequency multiplex techniques. The signals are amplified to a level at which the relative phase can be conveniently observed. Any difference in the timing of the processing of the signals comprises an error in the relative phase indication, thereby requiring that extreme measures be taken in the design of signal amplifiers to ensure that all signals suffer at least the same amplifier delays regardless of differences in amplitude, wave shape, noise level, etc. In precision type systems such as Differential OMEGA, ultimate accuracy can be limited more by the residual uncontrollable phase uncertainty in the receivers than by random propagation effects, particularly at short ranges.
In the prior art apparatus, variation in receiver delays between signals is eliminated from the phase indication by a technique involving cancellation of the received signals at the antenna terminals. A locally generated signal is injected into the antenna circuit and adjusted in phase and amplitude such that the phasor difference, as amplified to an observable level in a sensitive radio receiver goes to a null.